X-ray diffraction study of concentrated chromium (III) chloride solutions. I. Complex formation analysis in equilibrium conditions

Concentrated chromium (III) chloride solutions, both acidic and neutral, have been examined by x-ray diffraction in equilibrium conditions; i.e., a long period of time after their preparation. Complex formation between Cr3+ and Cl− ions have been quantitatively analyzed through direct comparison of experimental and calculated structure functions. The average number of chloride ions bonded to chromium in each solution has been determined. The values obtained are consistent with the indications given by the distribution curves and in good agreement with those expected on the grounds of equilibrium constants. The average aggregation numbers were determined by assuming the presence, in neutral solutions, of the charged species Cr(H2O)3+6, Cr(H2O)5Cl2+, and Cr(H2O)4Cl+2. In addition experimental evidence of the neutral species Cr(H2O)3Cl3 in the acidic solution has been obtained. Octahedral water coordination for chloride ions has been confirmed. Well defined second coordination shell around Cr3+ ions is strongly supported by the structural parameters obtained for water molecules belonging to two subsequent hydration shells of the cations.